Nitride free vapor deposited chromium coating

ABSTRACT

A process for applying a chromium layer on a substrate, specifically a turbine engine airfoil, by contacting at least a portion of the substrate with a gaseous chromium wherein the gaseous chromium is generated from a substantially nitrogen free source.

BACKGROUND OF THE INVENTION

This disclosure relates to improvements in forming chromium diffusioncoatings.

Substrates that are subject to corrosion, such as gas turbine enginecomponents, may include a coating to protect an underlying material fromcorrosion. Vapor deposition techniques can be used to deposit chromiumfor diffusion into the underlying material. However, typically thesetechniques use gases and activators that tend to include or formnitrides. Nitrides can have a negative effect on corrosion protectionand have to be removed prior to entry of the substrate into service.However, the processes used for the removal of the nitride layer candamage the substrates. The need exists for a way to address nitridedeposition issues without stressing and potentially damaging the parts.

SUMMARY OF THE INVENTION

According to the disclosure, a method is provided of applying a chromiumlayer on a substrate by diffusion wherein the layer is substantiallynitrogen free.

A process for applying a chromium layer on a substrate comprises:contacting at least a portion of a substrate with a vapor comprising agaseous chromium wherein the gaseous chromium is generated from amixture comprising a chromium source and a substantially nitrogen freeactivator.

In a further non-limiting embodiment of any of the foregoing examples,the chromium diffuses into the portion of the substrate to define thechromium layer.

In a further non-limiting embodiment of any of the foregoing examples,the mixture comprises a chromium solid.

In a further non-limiting embodiment of any of the foregoing examples,the contacting step takes place in an enclosed space.

In a further non-limiting embodiment of any of the foregoing examples,the mixture is heated to produce gaseous chromium.

In a further non-limiting embodiment of any of the foregoing examples,the substrate is an aircraft engine airfoil.

In a further non-limiting embodiment of any of the foregoing examples,the airfoil is a high pressure turbine stage airfoil.

In a further non-limiting embodiment of any of the foregoing examples,the airfoil is a low pressure turbine stage airfoil.

In a further non-limiting embodiment of any of the foregoing examples,conditions inside the enclosed space are between 1900° F. and 2000° F.

In a further non-limiting embodiment of any of the foregoing examples,conditions inside the enclosed space are held between 4 and 20 hours.

In a further non-limiting embodiment of any of the foregoing examples,the chromium layer has a thickness of between 0.3 and 1.3 mil(7.62-33.02 microns).

In a further non-limiting embodiment of any of the foregoing examples,the activator is a halide compound.

In a further non-limiting embodiment of any of the foregoing examples,the activator is a nitride free chloride compound.

In a further non-limiting embodiment of any of the foregoing examples,the activator is selected from the group consisting of chromium IIIchloride and chromium II chloride.

In a further non-limiting embodiment of any of the foregoing examples,the ratio of chromium to activator is between 120:1 and 160:1.

In a further non-limiting embodiment of any of the foregoing examples,the substrate has internal passageways.

In a further non limiting embodiment of any of the foregoing examples,the portion includes the internal passageways.

In a further non-limiting embodiment of the foregoing examples, thechromium layer covers the internal passageways.

In a further non-limiting embodiment of any of the foregoing examples,the substrate is made of a nickel-based superalloy.

In a further non-limiting embodiment of the foregoing examples, relativeto a total combined weight of the chromium, and the chromium chloride,the mixture has 98-99% by weight of chromium, and 1-2% by weight ofchromium chloride.

The details of one or more embodiments of the disclosure are set forthin the accompanying drawings and the description below. Other features,objects, and advantages of the disclosure will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 illustrates an example of a method of diffusion coating asubstrate.

FIG. 2 shows an example of a substrate that is prepared for diffusioncoating.

FIG. 2A shows a magnified section of the substrate with a chromiumlayer.

FIG. 3 shows a comparison of the resulting layers of conventionalchromium application and the current disclosure, with no nitride layerpresent in the latter.

DETAILED DESCRIPTION

According to the disclosure, a process is provided for applying achromium layer on a substrate comprising: contacting at least a portionof the substrate with a vapor comprising a gaseous chromium wherein thegaseous chromium is generated from a mixture comprising a chromiumsource and a substantially nitrogen free activator.

FIG. 1 illustrates an example of a method of diffusion coating asubstrate with a chromium layer, such as a substrate made of anickel-based superalloy. The method involves the use of the mixture thatis tailored to provide an effective chromium diffusion onto thenickel-based superalloy of the substrate without depositing nitrides onsaid substrate or the deposited chromium layer.

One advantageous embodiment of the mixture can consist essentially of anactive coating metal which can be a mixture of chromium, a diffusionactivator and a carrier material. For example, the mixture may includeonly the listed constituents and inadvertent impurities that do notinfluence the properties of the mixture. The chromium can be of flake orpowder form or consistency. In a preferred example chromium (III)chloride, which can be provided in a flake or powder form can be used asthe diffusion activator. In another example chromium (II) chloride,which can also be provided in flake or powder form can be used as thediffusion activator.

Other activators, such as other halide-based activators, vary ineffectiveness with regard to different active coating metals, includingchromium. However, the chromium (III) chloride, in the prescribedamount, is particularly effective for facilitating the diffusion ofchromium into nickel-based superalloys without forming nitrides on theresulting layer.

In a further example, the mixture, again relative to the total combinedweight, also has 98-99% by weight of chromium.

The amount of chromium (III) chloride diffusion activator and chromiumare selected in correspondence, for effective diffusion and areacontrol. The relationship between the amount of the chromium and theamount of the chromium (III) chloride can be represented as a ratio. Forinstance, mixture, relative to the total combined weight, can include X% by weight of the chromium and Z % by weight of the chromium (III)chloride in a ratio of X/Z that can be between 120:1 and 160:1.Providing the chromium and the chromium (III) chloride in the ratiodescribed ensures effective diffusion of chromium into the substrate andcontrol over the area of the substrate into which chromium diffuses.That is, the mixture herein limits lateral diffusion that wouldotherwise enlarge the coating area.

A variation of the method can include heating and application steps. Asan example, the heating is conducted in an enclosure having a continualflow of a carrying gas that is inert to produce an environment, in whichsaid inert gas is the most abundant gas, at a temperature greater than1900° F./1038° C., such as 1950° F./1066° C. to 2000° F./1094° C. Themixture can be heated until it starts vaporizing and is able to mix withthe carrying gas. In one embodiment of the method, the inert gas usedcan be Argon.

The application step includes contacting the gaseous mixture to thesubstrate, such as by introducing the substrate into a chamber filledwith the gaseous mixture. Alternatively, by heating the solid form untilthe solids vaporize to produce the desired gaseous mixture andsubsequently feeding the gaseous mixture vapor directionally onto thesubstrate. In another embodiment of the application the substrate canalso be heated prior to coating in order to enhance the receptionqualities of the substrate to chromium.

The substrate can be placed in contact with the gas for a selectedamount of time, depending upon a desired thickness of the chromiumlayer. In some examples, the selected amount of time is between 4 and 20hours and the final chromium diffusion coating (layer) includes at least20% by weight of chromium. As chloride compounds are contacted with thesubstrate, the aluminum in the substrate bonds to oxygen to createaluminum oxide and form the desired chromium layer. Thickness of theresulting chromium layer is dependent on the aluminum content of thesubstrate. In a preferred embodiment of the method, the resultingthickness of the chromium layer is between 0.3 and 1.3 mil (7.62-33.02microns).

FIG. 2 shows a substrate (1), which in this example is a gas turbineengine turbine blade. It is to be understood, however, that the presentmethod can also beneficially be applied to other substrates or other gasturbine engine components. In this example, the substrate generallyincludes an airfoil portion (3) and a root portion (5). The airfoil alsogenerally includes internal air passageways (7). In the preferredembodiment of the method the internal passageways also receive a layerof chromium. Further FIG. 2A shows a magnified version of the substrate(1) with a layer of chromium (17) present.

FIG. 3 shows a side by side comparison of the resulting layers ofchromium, one using the conventional method and the other using themethod of the present disclosure. Using the conventional method, theimage on the left, the resulting nitride layer (9) on top of thechromium layer (11) can be seen. The image of a substrate coatedaccording to the present disclosure, on the right, shows a chromiumlayer (13) on top of the substrate (15) with no nitride layer present.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The present disclosure provides a novel and non-obvious method for oneor more embodiments of the present disclosure. Variations andmodifications to the disclosed examples may become apparent to thoseskilled in the art that do not necessarily depart from the essence ofthis disclosure. For example, the exact ratios, enclosure conditions andthe initial state of chromium. Accordingly, other embodiments are withinthe scope of the following claims.

1. A process for applying a chromium layer on a substrate comprising:contacting at least a portion of the substrate with a vapor comprising agaseous chromium wherein the gaseous chromium is generated from amixture comprising a chromium source and a substantially nitrogen freeactivator.
 2. A process according to claim 1 wherein the chromiumdiffuses into the portion of the substrate to define the chromium layer.3. A process according to claim 1 wherein the mixture comprises achromium solid.
 4. A process according to claim 3 wherein the mixture isheated to produce gaseous chromium.
 5. A process according to claim 1wherein the contacting step takes place in an enclosed space.
 6. Aprocess according to claim 5 wherein conditions inside the enclosedspace are between 1900° F. and 2000° F.
 7. A process according to claim5 wherein conditions inside the enclosed space are held between 4 and 20hours.
 8. A process according to claim 1 wherein the substrate is anaircraft engine airfoil.
 9. A process according to claim 8 wherein theairfoil is a high pressure turbine stage airfoil.
 10. A processaccording to claim 8 wherein the airfoil is a low pressure turbine stageairfoil.
 11. A process according to claim 1 wherein the chromium layerhas a thickness of between 0.3 and 1.3 (mil) (7.62-33.02 microns).
 12. Aprocess according to claim 1 wherein the activator is a halide compound.13. A process according to claim 1 wherein the activator is a nitridefree chloride compound.
 14. A process according to claim 1 wherein theactivator is selected from the group consisting of chromium III chlorideand chromium II chloride.
 15. A process according to claim 1 wherein theactivator is chromium III chloride.
 16. A process according to claim 1wherein the ratio of chromium to activator is between 120:1 and 160:1.17. A process according to claim 1 wherein the substrate has internalpassageways.
 18. A process according to claim 17 wherein the portionincludes the internal passageways.
 19. A process according to claim 18wherein the chromium layer covers the internal passageways.
 20. Aprocess according to claim 1 wherein the substrate is made of anickel-based superalloy.
 21. A process according to claim 1 wherein themixture further comprises chromium (III) chloride.
 22. A processaccording to claim 20 wherein relative to a total combined weight of thechromium, and the activator, the mixture has 98-99% by weight of thechromium, 1-2% by weight of the chromium (III) chloride.